A. Technical Field
The present invention relates to energetic systems such as primers, igniters, and detonators, and more particularly to an energetic composite and system using an amassed plurality of energetic multilayer pieces which are formed from the division of a monolithic energetic multilayer, for enhancing the mechanical sensitivity of the energetic composite and system to initiation of self-sustained reaction.
B. Description of the Related Art
Many energetic systems such as primers, igniters, and detonators can be activated, i.e. ignited or detonated, via mechanical means, and as such are characterized as “impact initiated devices” or IID. Percussion primers used in small caliber (<20 mm) ammunition, and impact sensitive stab detonators used in medium caliber (20-60 mm) ammunition are two such examples.
Stab detonators in particular, such as illustrated at 10 in FIG. 1, are mechanically activated by stab initiation, a process by which a conical metal striker or firing pin 11 is mechanically driven (such as by a spring-loaded system) through a closure disc, cap, or seal 16 of an enclosure 12 and into at least a stab initiating mixture 13 (hereinafter “stab mix”) contained in the enclosure and comprised of energetic powders which are sensitive to mechanical stimulus. The mechanical energy of the driven firing pin is converted into heat through compression and friction of the stab mixture, with the rapid heating resulting in ignition of the mixture. The stab mix is typically the first level (i.e. the primary ignition material) of a detonator train, which is often a three-stage system but can also be either a two stage system or a greater than three stage system. FIG. 1 in particular illustrates a representative three-stage detonator train contained in the enclosure 12, comprising: the stab mix 13, a transfer charge 14, and an output charge 15 which is the main high energy explosive. Arranged in this manner, the rapid decomposition of the stab mix during ignition generates a pressure/temperature pulse that is sufficient to initiate the transfer charge, which has enough output energy to detonate the main output charge. It is appreciated that both the transfer charge and the main output charge may be characterized as being activated by the stab mix, i.e. the primary ignition material. Such stab detonators are typically very small in size (e.g. M55 stab detonator dimensions: 0.3 cm diameter, 0.4 cm length) and are used in a number of energy release train systems where weight and size limitations preclude the use of other types of initiators (heat or electrical).
It is appreciated, however, that energetic ignition mixes known in the art and used in a large variety of IIDs (such as the stab mix in stab detonators) are typically lead-based materials. For example, one common type of ignition mix known as NOL-130 is composed of lead styphnate (basic) 40%, lead azide (dextrinated) 20%, barium nitrate 20%, antimony sulfide 15%, and tetrazene 5%. These materials can pose acute and chronic toxicity hazards during mixing of the composition and later in the item life cycle after the item has been field functioned. Thus there is an established need to replace these lead-based mixes on toxicity, health, and environmental hazard grounds.
Energetic multilayer structures and nanolaminates are also known in the art having as small as atomic level layer thicknesses, such as disclosed in U.S. Pat. Nos. 5,538,795 and 5,547,715 to Barbee, Jr. et al, both of which are incorporated by reference herein. Such energetic nanolaminates are often energetic foils of metal multilayers, also known as “flash metal.” The exothermic reactions that are activated (such as by external mechanical input) in such energetic foils are the transformation of the multilayer material to its respective intermetallic alloy and the thermite reaction, which is characterized by very high temperatures, a small pressure pulse, and hot particle ejection. In particular, the energetic nanolaminates disclosed in the Barbee references are energetic multilayer flash metal foils capable of being prepared with tailored and precise reaction wave front velocities, energy release rates, and ignition temperatures. For example, the velocity of a multilayer thin film depends on the relative thickness and composition of each multilayer structure. Reaction front velocities from 0.2-100 meters/second can be prepared reliably and precisely. Multilayer reaction temperatures between 200 and 1500° C. are observed for multilayers with different compositional and structural characteristics. Heats of reaction from 0.1-1.8 kcal/g are capable with different multilayers. Various studies and reports are known which address the modeling and characterization of these properties and the influence of structure, composition, and processing conditions on such variables. Furthermore, the coating of sol-gels to multilayer energetic nanolaminates as energetic booster materials all also known, such as disclosed in U.S. Pat. Publication No. 2004/0060625 incorporated by reference herein, to further tailor reaction properties of nanolaminate igniters.
FIG. 2 illustrates a generic energetic nanolaminate construction, indicated at reference character 17, which is preferably a multilayer flash metal foil material that is periodic in one dimension in composition, or in composition and structure. They are fabricated by alternating deposition of two or more metallic materials. Individual layers can be varied in thickness from one atomic layer (˜2 Å) to thousands of atoms thick (>10,000 Å). The total thickness of the multilayer foil is shown as 20 in FIG. 2. And the period of the multilayer foil is the distance (i.e. thickness) of the repeating sub unit structure comprising two adjacent metallic layers, hereinafter referred to as the “bi-layer” (such as 18 in FIG. 2) that makes up the foil. It is notable that also included in each bi-layer is a pre-reaction zone (such as 19 in FIG. 2) which is the interface region between the adjacent layers of the multilayer and is made up of a thin layer of intermetallic product formed during deposition.
Multilayer structured materials can be formed by various different techniques known in the art. Physical vapor deposition, chemical vapor deposition, electrochemical deposition, electrolytic deposition, atomic layer epitaxy, mechanical deformation processing, etc. are all utilized to prepare multilayer materials. One type of physical vapor deposition involves sputtering. In sputter deposition systems atoms, or clusters of atoms, are generated in the vapor phase by bombardment of a solid source material with energetic particles. The substrate is moved past the source(s) and vapor condenses on the substrate to form a film. A single layer of material is deposited on the substrate with each pass. The thickness of component layers, and thus it's resulting physical properties, is precisely controlled by adjusting the periodicity of substrate movement. And magnetron sputtering is one type of sputtering technique and is the physical vapor method of choice for the semiconductor industry. Using magnetron sputtering techniques, alternating layers of different elements, each several nanometers thick, can be deposited on top of one another to make nanometer metallic multilayers with a thin intermixed region between the layers.
FIG. 3 illustrates the use of such known energetic nanolaminates, such as 22, in stab detonators, such as 21, as a replacement for the stab mix discussed above as the primary ignition material. Arranged as such adjacent to the closure disc 16 the energetic nanolaminates are penetrated by firing pin 11 to initiate self-sustained reaction of the nanolaminate. However, to be a suitable replacement for the stab mix of stab detonators, the energetic nanolaminate must be sensitive enough to be initiated with an input energy typically in the range of 0.5-5 in./oz. (3.5 to 35 mJ). Based on experiments performed by the Applicants at the Lawrence Livermore National Laboratory, however, monolithic energetic nanolaminates have been shown to require energy inputs of at least twice the maximum acceptable level for ignition.
There is therefore a need for a replacement stab mix with enhanced mechanical sensitivity level for use as the primary ignition material to initiate self-sustained reaction in stab detonators, such as for example, M55 and M61 stab detonators.